The phase locked loop concept has been used in a wide variety of applications including frequency synthesis, frequency multiplication, clock generation, and tone decoding. A phase locked loop is an oscillator that uses a feedback loop to lock the output frequency to an input reference. Phase locked loops can be used to generate stable frequency output frequencies from fixed low frequency reference signals.
Typically, a phase locked loop is a mixed signal application incorporating both analog and digital components. The basic operation involves comparing the phase of an oscillator output to the phase of a reference input signal using a phase detector. A signal representing the phase difference provides the control for the oscillator. A feedback path to the input of the phase detector ensures that the controlled oscillator output is “locked” to the frequency of the reference input signal.
Almost every electronic application is plagued by noise, unwanted signals that obscure the information carrying signal. Phase locked loop circuits are also sensitive to noise, particularly phase noise that causes errors or jitter in the frequency of the output signal. Noise may appear at the input of the phase locked loop and noise may be generated within the components of the phase locked loop as well.
When phase locked loops are used as frequency synthesizer components in RF communication systems, for example, noise associated with the frequency synthesizer directly degrades the performance of the entire system. In addition, noise in phase locked loops affects the timing margins of the system design. Phase locked loop communication applications sensitive to noise may include those involving long distances or a series of cascaded phase locked loops. In these applications, controlling the noise introduced in each phase locked loop stage may be especially critical.
As system speeds increase, more stringent noise requirements are imposed on phase locked loop circuits because higher frequency operations cannot tolerate the same amount of noise and associated jitter. These and other difficulties present challenges to the implementation of noise control in phase locked loop circuits.